This invention relates to a transceiver circuit, a target detection apparatus using such a transceiver circuit and a method of operating a transceiver circuit.
It is well known to use target detection systems such as radar systems in order to detect targets around an apparatus. Typically, radiation such as radio waves will be transmitted from a transmitter and received at a receiver, and by comparing what is transmitted with what is received, a determination is made as to the presence and potential position of targets around the apparatus.
An example prior art frequency modulated continuous wave radar apparatus is shown in FIG. 1 of the accompanying drawings. It comprises an oscillator 1, which produces a signal having a modulated frequency. An example of a frequency modulation pattern used is shown in FIG. 2a of the accompanying drawings, which shows in solid lines the frequency of the signal generated with time.
This signal is passed to a transmitter 2, which generates radio waves and transmits them in the direction of an area where there may be targets 3. A receiver 4 receives reflected radiation that has been reflected off any targets 3, with a time delay that depends on the range of the target. However, because no circuit is perfect some of the transmitted signal bleeds through to the receiver. Furthermore, due to very short range internal reflections within the radar apparatus or within the vehicle in which it is mounted, some of the output signal is reflected internally back to the receiver. Thus, a proportion—the unwanted portion—of the output signal is received directly at the receiver 4 with a range of very short time delays. This unwanted portion is shown in dotted lines in FIG. 2a. 
The transceiver also has a mixer 5, which mixes the output and received signals. As is well known, this produces a mixed signal which will contain components at the sum and the difference of the frequencies of the output and received signals. In order to analyze only the difference signal, a low pass filter 6 is provided, which discards those components of higher frequency than the expected difference components, and a high pass filter 7, to block the DC component of the mixed signal, to produce a filtered signal, shown in the ideal case in FIG. 2b. 
However, the unwanted portion will also be included in the received signal which is mixed with the output signal. Because of the short time delays, this means that, where there are discontinuities in the frequency modulation of the output signal (and particularly if there are steps in frequency), there will be large discontinuous components at relatively high frequency in the mixed signal, and response to these discontinuous components thereafter in the filters 6, 7. These are shown labeled with arrows in FIGS. 2a and 2b. 
As such, this can lead to deleterious effects in the high pass filter 7. Most practical high pass filters will have a transient response to sudden discontinuous frequency spikes such as are shown in FIG. 2b. This can be seen in FIG. 3 of the accompanying drawings, where the smoother trace shows the effects of such transients on the filtered signal, and the less smooth trace shows the underlying filtered signal. As such, it can be seen that the transients are low frequency compared with the underlying filtered signal, but with an amplitude several times that of the underlying signal.
One previous proposal to ameliorate for this problem has been to correct for the transient effects of the unwanted portion in the frequency domain. Typically, in most radar systems, a Fast Fourier Transform (FFT) will be taken of the filtered signal, after it has been digitized by an analogue to digital converter 8. This is done in this proposal. At this point, a correction is stored in memory based upon a known response of the filters 6, 7 to a known output signal and then a subtraction from the FFT in the frequency domain.
This can be seen in FIGS. 4a and 4b of the accompanying drawings, where FIG. 4a shows the FFT having been taken before any correction is applied, and FIG. 4b shows the FFT after the correction is applied. The difference in the vertical scale between the two graphs should be noted; given the low frequency character of the transient response, the subtraction is largely applied to the lowest frequency components of the FFT. Whilst this provides useful data, it loses much data that was present in the lower frequency components, which will relate to short-range targets. This effectively restricts the minimum range at which the radar system can operate.
Another alternative proposal to ameliorate for the transient response of the filters to the transients in the mixed signal is to calculate or measure the response of the filters to a given output signal unwanted portion and to save that waveform in memory. However, to record the response of the filters to all the different transients, particularly on many channels, can be inefficient use of memory.
One yet further approach is to create an additional bleedthrough signal path in the RF circuitry with 180 degree phase shift which cancels the unwanted bleedthrough signal before arriving at the filters. This can only deal with the bleedthrough source, not other unwanted reflections (e.g. from parts of the vehicle to which the radar system is mounted).
Thus, it would be desirable to provide a transceiver circuit capable of correcting for undesirable signals received by the receiver.